Method for treating process condensate

ABSTRACT

An apparatus and method for treating chemical production plant process condensate such that a contaminant-rich stream and a relatively pure aqueous stream is separately recoverable from the condensate, wherein the contaminants are substantially removed from the condensate by steam stripping and subsequent rectification in a relatively low pressure stripping/rectification tower. The tower overhead is then condensed, and any non-condensed gases are subjected to water scrubbing to further recover contaminates from the non-condensed gas. A portion of the condensed overhead and scrubbing water containing contaminates is returned to the top of the rectification section of the tower as reflux and the balance being withdrawn as a concentrated stream for reuse in the plant. The apparatus may be used in conjunction with existing low pressure equipment, avoiding costly major modifications, and is particularly adapted to use in conjunction with ammonia and methanol plants.

This application is a Continuation-in-Part of U.S. patent applicationSer. No. 08/323,855 filed Oct. 17, 1994 which is now issued as U.S. Pat.No. 5,498,317, which is a Divisional application of U.S. Ser. No.08/116,863 filed Sep. 3, 1993 which has now issued as U.S. Pat. No.5,385,646.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to industrial chemicalproduction plants and waste water treatment, and is more specificallydirected to a novel apparatus configuration and method for using thesame to recover raw materials, by-products and product from the diluteprocess condensate streams of chemical production plants. The recoveredmaterials are recycled for use in the production facilities in such amanner as to avoid any significant energy or other efficiency penaltieswhich could negatively impact the plant's operation and overalleffectiveness.

2. Description of the Related Art

Large quantities of industrial waste water are daily produced bychemical production and processing plants within the United States andthroughout the world. Often times, this waste water is processcondensate consisting of dilute streams of raw materials, byproducts andproduct remaining unrecovered from processing water and/or steam used invarious phases of production. For example, in the production of ammonia,steam exiting the plant after use in stripping operations carries traceamounts of methanol, ammonia, carbon dioxide, alkylamines and the like.Although the materials within these streams could be utilized to formproduct, due to the dilute nature of the condensate, it is generallymore cost efficient to simply consider the water as waste water anddispose of the same as needed.

At one time, the bulk of process condensate and industrial waste waterwas simply discharged into live streams or municipal sewer systemswithout treatment. However, in view of the potential environmentaldamage that could result from the release of chemicals into the watersystems, as well as the need to conserve the amount of water used dailyin operations, methods have been developed for treating the water toremove any contaminants therefrom and recycle the water back to phasesof the plant for reuse.

In the field of ammonia production, for example, it is known to use arelatively low pressure steam stripping apparatus such as a conventionalstripping tower to treat the process condensate, wherein steam isutilized to strip the contaminates from the condensate. The contaminatedoverhead is then vented to the atmosphere while the stripped condensateis reused in the plant as cooling tower water make-up, boiler feed watermake-up and the like. Alternatively, the contaminated overhead may bedestroyed or decomposed such as by burning. Although these conventionallow pressure strippers are useful for this purpose, the amount ofcontaminants and/or noxious vapors vented to the atmosphere isundesirable. In fact, the type and amount of such emissions is thesubject of increasingly stringent regulation by the EnvironmentalProtection Agency (EPA), as well as other state and local officials. Itis anticipated that the level of emissions now permitted will besubstantially reduced in the future, particularly as to potentiallyharmful compounds such as ammonia and methanol, and will perhapseventually be prohibited altogether.

In order to overcome these emission problems, a more recently developedmethod of treating ammonia plant process condensate utilizes relativelyhigh pressure condensate stripping towers, generally operating at 500pounds pressure (psi) or more to strip contaminants from the condensate.In this method, the high pressure process steam carries the contaminatedstripper overhead back to the plant for use in the primary reformerstage of production. Although return of the contaminates to the ammoniaplant avoids undesirable venting of contaminants into the atmosphere,injection of the relatively high pressure stream of overhead into theplant requires that the flow of this high pressure stream into the plantbe controlled and that this additional stream be taken into accountalong with the normal steam flow in setting the steam-to-gas ratio. Thishas the effect of lowering the front-end pressure of the system,lowering ammonia plant capacity and efficiency as well as complicatingprocess control overall. In addition, the high pressure strippers arerelatively very expensive and their use requires the completereplacement of the more conventional low pressure strippers that arepresently use3d in many plants throughout the world.

Therefore, it is a primary object of the present invention to provide anapparatus and method of treating chemical plant process condensate,wherein the raw materials, product and byproducts (hereinafter referredto collectively as "contaminants") in the condensate are recovered fromthe water for reuse in the plant.

It is another object of the present invention to provide an apparatusand method of treating process condensate, wherein contaminants in thecondensate can be recovered and returned to the plant without accruingsignificant efficiency penalties to the plant or requiring plantmodification.

It is another object of the present invention to provide an apparatusand method of treating process condensate, wherein a conventional lowpressure steam stripping tower may be utilized.

Another object of the present invention is to provide an apparatus andmethod of treating process condensate, wherein a plant using aconventional low pressure steam stripping tower for condensate treatmentcan be retro-fitted to enable the concentrating and recycling ofrecovered contaminants back to the plant.

It is another object of the present invention to provide an apparatusand method of treating process condensate that is relatively efficientand cost effective.

A further object of the present invention is to provide an apparatus andmethod of treating process condensate wherein contaminant air emissionsare effectively eliminated.

Yet another object is to provide an apparatus and method of treatingprocess condensate, wherein a substantial amount of the water in thecondensate can be recovered having a reduced contaminant content so asto be suitable for reuse in the plant as boiler feedwater make-up whichrequires high quality, low conductivity water.

Yet a further object of the present invention is to provide an apparatusand method of treating ammonia and/or methanol plant process condensatemeeting the objectives heretofore described.

SUMMARY OF THE INVENTION

These and other objects are achieved by a novel apparatus configurationand method of using the same to recover product, by-products and rawmaterials from the process condensate of a chemical production plant.The method comprises stripping the contaminants from the condensate in arelatively low pressure stripping section of an upright tower to obtaina contaminant-rich vapor and an aqueous bottom stream of reducedcontaminant content. This vapor is then rectified in a rectificationsection of the tower to obtain a concentrated overhead stream. Theoverhead is then condensed and any remaining vapor is passed through avapor scrubber to recover any trace amounts of contaminates intocondensed form. A portion of the condensed overhead stream is thenreturned to the top of the rectification section of the tower as reflux,and the balance is withdrawn as a concentrated stream for re-use in theplant.

Due to the low volume nature of the concentrated overhead streamwithdrawn, the contaminants can be efficiently injected back into theplant at the appropriate stage for decomposition, recycling and/or reusewithin the plant without significant thermal, pressure or energy impacton the plant and its operation. Thus this process does not requiresignificant plant modification and does not accrue significantefficiency penalties. As with other stripping operations, the strippedcondensate is removed from the tower as bottoms liquid which may also berecycled for use as boiler feed water, cooling tower water make-up, orother suitable purposes.

In an alternative embodiment, separate stripping and rectificationcolumns operating in series are provided, whereby the overhead vapor ofthe stripping column is delivered to the base of the rectificationcolumn. The rectification bottoms are returned to the top of thestripping tower for further stripping action. The concentrated overheadfrom the rectification column is then condensed and scrubbed, with aportion of condensed and scrubbed overhead being returned to the top ofthe rectification column as reflux and the remainder being withdrawn asa concentrated stream for recycling to the chemical production plant.

The method and apparatus configuration of this invention can be utilizedin conjunction with conventional low pressure steam strippers, such asthose previously used for treating ammonia plant or methanol plantprocess condensate, by retrofitting the existing stripping tower toinclude a rectification section at its top or by adding a separaterectification column in series. Not only does the retrofitted systemeliminate the environmental concerns associated with the prior techniqueof venting contaminates, it enables substantially the whole of theprocess condensate contaminates generated by the plant to be convertedinto feed stock which can be recycled to the production facility foruse.

This apparatus and method achieve a marked improvement in the overalleconomics and operation of the chemical production plant complex, makingit possible to obtain the advantages now associated with relatively highpressure strippers while avoiding the enormous costs associated withsuch systems, and the energy and pressure penalties which necessarilyaccrue to the production plant via their use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an apparatus configurationhaving a common tower for stripping and rectification in accordance withthe present invention;

FIG. 2 is a schematic representation of an apparatus configurationhaving separate stripping and rectification columns in accordance withthe present invention;

FIG. 3 is a schematic representation of a preferred embodiment of theinvention of FIG. 1, wherein the apparatus is provided for the treatmentof ammonia plant process condensate;

FIG. 4 is a schematic representation of a further embodiment of theapparatus configuration of FIG. 1, wherein a vent scrubber having anextended sump is utilized to recover contaminants from any remainingvapor in the condensed overhead;

FIG. 5 is a schematic representation of a portion of the apparatus ofFIG. 1 including a vent gas scrubber directly flanged to the overheadreceiver; and

FIG. 6 is a schematic representation of a portion of the apparatusconfiguration of FIG. 1 wherein a separate vent scrubber is connected byflow lines to the overhead receiver.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a novel method and unique apparatusconfiguration for treating chemical production plant process condensate.The process condensate treated may generally include any effluentgenerated by chemical production and processing operations, and willnormally comprise a dilute aqueous stream of the raw materials utilizedin processing, as well as product and by-products formed in variousstages of production. For purposes of this application the product,by-product and raw materials contained in the condensate shall bereferred to collectively as "contaminants". Depending upon the nature ofthe processing steps from which the condensate is generated, thiseffluent may be presented for treatment in the form of a gaseous orliquid stream. The process condensate to be treated may be used at itsrecovery temperature without preliminary treatment and will preferablyhave a temperature ranging near its bubble point for the strippingoperation pressure as hereafter described.

Referring now to the embodiment shown in FIG. 1, a process condensatestream from a chemical production plant is fed via line 2 to an entryport 4 in the midsection of an upright cylindrical tower 6 having aclosed top and bottom. Tower 6 comprises a lower stripping section 8 andan upper rectification section 10. The tower is of conventionalconstruction, preferably a countercurrent tower of the bubble-plate orpacked type operable at relatively low pressures ranging generally from0 psia to 300 psig and most preferably ranging from about 0 psia to 100psig.

Although any low pressure relatively inert stripping fluid is consideredsuitable for purposes of this invention, it is suggested to use upwardlyflowing steam as the stripping gas. Steam may be supplied under lowpressure through a line 12 to the tube side of a reboiler 14 generatingsteam that is delivered to the base of the tower via line 16.Alternately, the steam may be directly injected into the base of tower 6(not shown).

Upon entering port 4, the process condensate flows downwardly throughstripping section 8 and at least a portion of the contaminants arestripped from the condensate by steam vapor rising countercurrentlythrough this section. The stripped condensate collected at the closedbottom of tower 6 flows to the shell side of reboiler 14 by means ofline 18 and is vaporized generating steam flow to the tower via line 16.The portion of the bottoms liquid not vaporized is withdrawn from thereboiler by means of a bottoms pump 20 and is discharged along line 22for use in various stages of chemical production. For example, thebottoms liquid of reduced contaminant content may be used as boiler feedwater make-up. Alternatively, the stripped condensate can be used forcooling tower water make-up or may be directly discharged under permitinto the municipal sewer system or local waterways with little or nofurther treatment.

The stripped contaminates in vapor pass upwardly within the tower 6through rectification section 10. Some of the vapor is recondensedduring rectification and flows downwardly within the tower to thestripping section 8 below. The remaining vapor is withdrawn from the topof tower 6 as a concentrated overhead stream. The overhead is deliveredalong line 24 to condenser 26 whereby the overhead is totally condensedand subcooled. Suitable condensers include, but not by way oflimitation, heat exchangers, water coolers and/or air fin coolers forpurposes of this invention.

The condensate is then transferred along line 28 to overhead receiver 30which is preferably a pressure vessel receiver such as that kindconforming with ASME standards. Traces of non-condensables such asdissolved hydrogen gases, for example, may be vented from receiver 30 tothe atmosphere along line 32. During start-up of the treatment process,the condensed overhead stream is withdrawn from receiver 30 along line34 via reflux pump 36 whereby all of the condensed overhead is returnedalong line 38 as reflux to the top of tower 6. Although any conventionalpump may be used for this purposes, an ANSI standard end suctionvertical centerline discharge pump such as that available from GouldsPumps, Inc. out of Seneca Falls, N.Y. or that offered under thetradename Durco™ from The Duriron Company Inc. out of Dayton, Ohio isconsidered particularly suited to this invention.

Once the contaminates reach concentrated levels within the condensedstream, preferably comparable to about a 30 to 200 fold increase inconcentration over that amount in the process condensate fed to thetower along line 2, the condensed overhead stream is split such that aportion of the stream is withdrawn along line 40 at a controlled rate toprovide a withdrawn stream of much greater contaminant concentrationthan the condensate feed. Note that this level of concentration in thewithdrawn stream is about 5 to 15 times greater than it is in theoverhead vapor from a process condensate stripper with no rectification.To meet this objective, the split stream is generally withdrawn at acontrolled rate ranging from 3 and up to about 50% by volume of thetotal condensed stream. Preferably, the condensate will be withdrawnalong line 40 at a rate of about 5 to 20% and most preferably about 10to 15% by volume of the total stream so as to correspond to a refluxratio ranging anywhere from 6:1 to 10:1 reflux to withdrawn condensate.

In this manner, the concentration level of the contaminants in thewithdrawn stream is so high (having a reduced overall liquid volume)such that the thermal load transferred to the plant upon injection ofthe withdrawn steam back into the production plant is relatively minorin comparison to what it would be if the stripping tower overhead weretotally condensed and injected directly into the plant withoutconcentration (i.e. rectification). The withdrawn concentrated overheadis then recycled back into the plant along line 40 via injection pump 42at the appropriate stage of the plant's operation so that thecontaminants may be decomposed, reused and/or recycled for theproduction of chemical product.

In an alternative embodiment as shown in FIG. 2, the stripping andrectification steps of the present invention are conducted in twoseparate columns presented in series. In this embodiment, the processcondensate is first introduced along entry port 4 to the top ofstripping column 108 and later rectified in separate rectificationcolumn 110. Each column 108 & 110 is made of conventional constructionas heretofore described preferably being of the cylindricalcountercurrent bubble-plate or packed type and operable at relativelylow pressure. This embodiment is particularly well adapted for use inretrofitting existing low pressure steam stripping operations whereinthe stripping tower is relatively small such that a rectificationsection cannot be fitted within the existing tower.

As in the first embodiment, steam may be supplied under low pressure tothe stripping column 108 through a line 12 to the tube side of areboiler 14 vaporizing water generating steam that flows to the base ofthe column via line 16. Alternately, the steam may be directly injectedinto the base of stripping column 108 (not shown). Upon entering port 4,the process condensate flows downwardly through stripping column 108 andat least a portion of the contaminants are stripped from the condensateby the steam vapor rising countercurrently through the column. Thestripped condensate collected at the closed bottom of stripping column108 flows to the shell side of reboiler 14 by means of line 18 where itis vaporized to steam that flows to the tower via line 16. The portionof the bottoms liquid not vaporized is withdrawn from the reboiler bymeans of a bottoms pump 20 which is discharged along line 22 for use invarious stages of the production facilities.

The contaminant-rich vapor overhead from stripping column 108 is thensupplied via line 109 to the base portion of rectification column 110.Some of the vapor is condensed during rectification and, along withreflux, subsequently flows downwardly through the column 110 to providea bottoms liquid. The bottoms liquid is withdrawn through a line 112 viapump 114 for delivery to the top of stripping column 108 for additionalstripping action. The remaining vapor is withdrawn along line 24 as aconcentrated stream for subsequent condensation and use as more fullydescribed above in conjunction with the first embodiment.

Another embodiment of the invention is specifically directed to use ofthe present apparatus and method of using the same for treating ammoniaplant process condensate. In this embodiment, the process condensategenerally comprises condensate from the reforming stages of ammoniaproduction, water formed in CO shift converters, and secondarycondensate recovered from carbon dioxide stripping operations within theplant such as from a Benfield hot potassium carbonate CO₂ removalsystem. The process condensate will include ammonia in an amount ranginganywhere from 500 to 2,000 parts per million (ppm) by weight of thecondensate, methanol in amounts ranging from about 100 to 800 ppm byweight of the condensate, as well as trace amounts of other by-products,raw materials and impurities such as alkylamines and dissolved nitrogenand hydrogen gas. The process condensate may be utilized for purposes ofthis invention at its recovery temperature (recovery from the plant)which generally ranges from 150° to 250° F. and is most preferably at atemperature near the bubble point of the condensate for the toweroperation pressure.

A further preferred embodiment of the invention is shown in FIG. 3.Looking to FIG. 3, the ammonia plant process condensate is fed via line2 to an entry port 4 in the midsection of an upright cylindrical tower 6comprising a lower stripping section 8 and an upper rectificationsection 10. The tower is of conventional construction as heretoforedescribed in the first embodiment operable at relatively low pressuresranging generally from 0 psia to 300 psig and most preferably rangingfrom about 10 to 100 psig. Steam is supplied under low pressure througha line 12 to the tube side of a reboiler 14 with flow to the reboilerbeing controlled by flow control valve 115 generating steam in thereboiler which is delivered to the base of the tower via line 16.

Upon entering port 4, the process condensate flows downwardly throughstripping section 8 and the contaminants are stripped from thecondensate by steam vapor rising countercurrently through this section.The stripped condensate collected at the closed bottom of tower 6 flowsto the shell side of reboiler 14 by means of line 18 and is vaporizedgenerating steam to the tower via line 16. A portion of the bottomsliquid is withdrawn from the reboiler controlled by a level controlvalve 118 by means of a bottoms pump 20 which is discharged along line22 for use as boiler feed water make-up or cooling tower water make-upin the ammonia plant.

The stripped contaminate-rich vapor passes upwardly within the towerthrough rectification section 10. Some of the vapor is recondensedduring rectification and flows downwardly within the tower to thestripping section 8 below. The remaining vapor is withdrawn from the topof tower 6 as a concentrated overhead stream. During initial start-up ofthe system, the overhead from tower 6 is diverted along line 120 andvented to the atmosphere until the pressure and composition of theoverhead stream approach design conditions within the rectificationsection 10 of tower 6. This pressure is controlled by pressurecontroller 122 which is connected to valves 124 and valves 126 such thatuntil equilibrium is reached, the overhead will flow along line 120 withflow via line 24 to the condenser blocked by valve 128. Once equilibriumis reached, line 120 is closed and line 24 opened.

The overhead is then delivered along line 24 to first condenser 26awhereby the overhead stream is totally condensed. First condenser 26a isin the form of a heat exchanger, wherein the preferred coolant is adilute solution of ammonium nitrate which upon heating will then bedelivered to a concentrator (not shown) in the ammonia plant. A completedescription of such a concentrator system appears in Holiday, SystemCurbs Nitrogen in Plant-Effluent Streams, CHEMICAL ENGINEERING (Aug. 14,1978) incorporated herein by reference. In this embodiment, the wastewater stream from the concentrator is thus used for heat exchange withthe tower overhead stream and the heated cooling water is then recycledalong line 130 back to the concentrator for reuse. Although direct heatexchange is contemplated for use in this embodiment, indirect heatexchange is deemed suitable for these purposes and may be advisable incertain circumstance to eliminate the risk of leakage of the waste waterstream into the overhead stream. An indirect heat transfer system wouldof course require additional equipment such as an additional holdingtank, fluid pump and heat exchanger.

It should be understood that the cooling fluid stream of the heatexchanger could have its source in any number of plant operations,whereby the heated stream may be returned for recycled use in the plant.Alternately, cooling tower water can be used in the heat exchangers tocondense the overhead. In another alternate condensing mode, an air fincondenser could be used.

After condensing, the condensate is directed along line 132 and isfurther subcooled to reduce the vapor pressure in trim cooler 26bpreferably to a temperature ranging from 100° to 150° F. with a vaporpressure ranging from 2 to 8 psia. The subcooled condensate is thendirected along line 28 to receiver 30 wherein normally closed valve 134may be opened to allow traces of non-condensables in the condensedoverhead to be vented to the atmosphere. This valve may be operatedmanually or by monitoring the pressure within the receiver. Thecondensate is withdrawn from the receiver 30 in conjunction with levelcontrol valve 136 at a rate corresponding with the level of fluidcollected within the receiver.

In the embodiment shown in FIG. 3, a meter 138 is provided along line 34to monitor the content of the condensed overhead stream as it exits theoverhead receiver 30 to assure that the condensed stream is free of anyunwanted contaminates. For example, ammonium nitrate contained in theheat exchanger loop described above could potentially leak into thestream during heat exchange. The presence of such ammonium nitrate couldbe detrimental to the operation of the ammonia plant if recycled to theplant as is contemplated by this invention. Thus a meter of any typedeemed suitable for purposes of recognizing unwanted contaminates isconnected to diversion valve 140 which is normally closed. Should anyunwanted contaminates be present in the condensed stream, the streamwill automatically stop feeding line 40 and instead be diverted alongline 142 to a sump or other waste containment region. In this manner,the integrity of the ammonia plant process is not in any way jeopardizedby the injection of the condensed stream into the ammonia plant. In apreferred embodiment, when the heat exchanger coolant above is providedfrom the ammonia plant concentrator, meter 138 is an ion specificelectrode for nitrate.

Initially, all of the condensed overhead withdrawn from receiver 30 isreturned to the top of tower 6 as reflux until the contaminates reachconcentrated levels within the condensed stream, preferably comparableto about a five to fifteen fold increase in concentration over thatamount of contaminants that would be in a conventional stripper overheadsteam vent. This concentration is about 40 to 120 times greater than inthe process condensate feed to the tower. The condensed overhead streamis then split such that a portion of the stream is withdrawn along line40 preferably at a rate of about 5 to 20% and most preferably about 10to 15% by volume of the total stream so as to correspond to a refluxratio ranging anywhere from 6:1 to 10:1 reflux to withdrawn condensate.As is shown in Table I below, in a most preferred embodiment of theinvention, the condensed stream is withdrawn at a reflux rate of about8:1 reflux to withdrawn condensate.

                                      TABLE I                                     __________________________________________________________________________    EXAMPLE MATERIAL BALANCE OF AN OPERATING CONDITION                            USING THE APPARATUS CONFIGURATION OF FIG. 3                                             A   B   C    D    E    F                                            __________________________________________________________________________    Water, Lbs/Hr                                                                           149,740                                                                           32,700                                                                            24,676.29                                                                          21,934.48                                                                          2,741.81                                                                           146,998.19                                   Ammonia, Lbs/Hr                                                                            225   2,023.65                                                                           1,798.80                                                                            224.85                                                                               0.15                                     Methanol, Lbs/Hr                                                                           35     300.06                                                                             266.72                                                                             33.34                                                                                1.66                                     Total, Lbs/Hr                                                                           150,000                                                                           32,700                                                                            27,000                                                                             24,000                                                                             3,000                                                                              147,000                                      Pressure, PSIG                                                                             28                                                                                50                                                                              29   60  650   30                                          Temp, °F.                                                                           238                                                                              297                                                                             234  120  120   274                                         __________________________________________________________________________

                  TABLE II                                                        ______________________________________                                        PERFORMANCE FOR VARIED OVERHEAD                                               WITHDRAWAL RATES CALCULATED FOR                                               APPARATUS CONFIGURATION OF FIG. 3                                             ______________________________________                                        Overhead Withdrawal Rate #/Hr.sup.1                                                                  3000   917                                             Reflux Ratio           8.0    27.5                                            NH.sub.3 in Withdrawn Overhead                                                                       7.5    24.5                                            MeOH in Withdrawn Overhead                                                                           1.1     3.6                                            NH.sub.3 in Bottoms, ppm.sup.2                                                                       1.0     1.75                                           MeOH in Bottom, ppm.sup.2                                                                            11.3   14.1                                            % of Flood              80     80                                             Theoretical Stages      12     12                                             ______________________________________                                    

The withdrawn concentrated overhead is the recycled back into theammonia plant along line 40 via injection pump 42 at different stages ofthe ammonia production operation. Although it should be understood thatthe concentrated overhead stream may be utilized in any manner deemedsuitable, in one embodiment of the invention the concentrated stream isfed along line 40 and injected to the gas and steam mixed feed coil ofthe primary reformer or to the air preheat coil of the secondaryreformer of the plant. In any event, in order to eliminate any pressurepenalties on the ammonia plant, the concentrated stream should beinjected back into the plant under pressures on the order of 500 to 600psi, or to any other pressure equivalent to the pressure of the streamin which it is placed.

For these purposes, injection pump 42 is preferably a high differentialpressure positive displacement reciprocating pump having a variablefrequency motor such those manufactured by Union Pump Company out ofMichigan, Wilson Snyder Pumps out of Texas or Milton Roy Co. out ofPennsylvania. These pumps are preferred to assure that the condensedoverhead is efficiently withdrawn at a relatively low rate in accordancewith the present invention, and delivered to the ammonia feed coil atrelatively high pressures. Of course a high speed centrifugal pump couldalso be used for purposes of this invention, but may be less efficientin view of the low rate of withdrawal required.

In yet another embodiment of the invention, it has been found that thepercentage of contaminates recovered from the process condensate can beincreased by scrubbing any non-condensed vapors remaining aftercondensing the overhead withdrawn from the stripping/rectificationtower(s). In this manner, trace amounts of ammonia, methanol, and othercontaminates contained within the non-condensed vapors are recovered forre-use in the plant. This may be accomplished as shown in FIG. 4,whereby the overhead receiver is replaced by a vent gas scrubber 31having an extended sump.

In practice, overhead vapor is withdrawn from the top of tower 6 alongline 24 and is condensed in condenser 26. The condensed overhead and anyremaining non-condensed vapors are then delivered to the base orextended sump of vent gas scrubber 31. The non-condensed vapors passupwardly through the scrubber countercurrent to the flow of scrubbingwater which is supplied to the top of the scrubber along line 23. Asshown in FIG. 4, the scrubbing water may be supplied by the bottomsliquid removed from the bottom of tower 6. Contaminates within the vaporare recovered in the scrubbing water and collected at the base of thescrubber as condensed overhead. The remaining non-condensable gases arethen vented to the atmosphere through vent 32. The condensed overhead iswithdrawn along line 34 and further processed in accordance with theinvention as previously described.

In an alternative embodiment shown in FIG. 5, a conventional vent gasscrubber 131 is directly flanged to overhead receiver 130. The condensedoverhead and any remaining non-condensed vapors after the condensingstep are delivered to receiver 130 along line 28. The non-condensedvapors flow upwardly through the scrubber countercurrent to the flow ofscrubbing water which is delivered to the top of the scrubber along line23. Contaminates within the vapor are recovered within the scrubbingwater and collected in overhead receiver 130 as condensed overhead. Anyremaining non-condensables are vented along line 32 to the atmosphere.The condensed overhead collected in overhead receiver 130 is withdrawnalong line 34 and further processed in accordance with the invention asearlier described.

In still another embodiment shown in FIG. 6, an existing processcondensate treatment facility in accordance with this invention may beretrofitted by adding a vent gas scrubber 231 in fluid flow contact withoverhead receiver 30. In this embodiment, the condensed overhead and anyremaining non-condensed vapors are transferred from the condenser alongline 28 to overhead receiver 30. The non-condensed vapors flow from thetop of overhead receiver 30 along line 29 to the base of vent gasscrubber 231. Scrubbing water is delivered to the top of the scrubberalong line 23. The non-condensed vapors flow upwardly through thescrubber countercurrent to the flow of scrubbing water, and contaminateswithin the vapor are recovered by the scrubbing water. The contaminatedscrubbing water flows downwardly through the scrubber and is withdrawnalong line 33. The contaminated scrubbing water is then joined withcondensed overhead withdrawn from overhead receiver 30 for furtherprocessing along line 34 in accordance with this invention.

It should be understood that during the practice of this invention,various systems and apparatus can be employed to monitor and control therate of flow of the processed streams and the temperature and vaporpressure of these streams. Such control systems may be based upon valveoperations as shown in FIGS. 3, 5, and 6 hereof, by computercalculations and/or manual adjustments.

The apparatus used for purposes of the invention may generally becomprised of any relatively chemically inert, durable materials such ascarbon steel, stainless steel, certain polymers and metal alloys knownin the art. It should be understood that the apparatus including thetowers, pumps, condensers, valves, receivers, and boilers areconventional and may be generally dimensioned to meet that task at handtaking into consideration the volume of vapor and liquid flow beingprocessed and size of the ammonia processing plant.

The invention is further illustrated by the following example which isillustrative of certain embodiments designed to teach those of ordinaryskill in the art how to practice the invention.

EXAMPLE 1 RETROFITTING EXISTING LOW PRESSURE STRIPPING OPERATION

A conventional low pressure stripping system for treating processcondensate from an ammonia plant was retrofitted in accordance with thepresent invention. The existing system comprised a stripping tower of acarbon steel column design by Chemical Construction Company, having aninternal design pressure of 155 psig at 450° and being 5' 6" in diameterby 54' by 10" tangent to tangent for condensate stripping. The tower waspacked with two sections of one inch Flexirings™ packing, a product ofKoch Engineering Company out of Wichita, Kans., wherein the bottompacked section of the tower had 20' depth and the top packed section was18'2" in depth. Other tower internals including packing bed supports,liquid distributors, packing hold downs and demisters. A used kettletype reboiler was placed into service to indirectly provide strippingsteam. The column boil up rate was controlled by steam to the reboileron flow control. Column pressure control was by a pressure control valveon the overhead vapor. The overhead vapor containing steam, ammonia,methanol, and traces of alkylamines was vented to the atmosphere.

The stripping tower was revamped in accordance with the processcalculations for the hydraulic loads appearing in Tables I and II aboveindicating that twelve theoretical stages were required to accomplishthe flow sheet separation with the reflux ratio desired. Approximatelytwo stages were needed in the enriching rectification section and tenwere deemed required in the stripping section. The height equivalent toa theoretical stage or plate, HETP, was calculated to be about 2.5 to3.0 feet based on actual column process performance data with one inchFlexirings™ in a stripping configuration. This resulted in a retro-fitprocess design of 30' of one inch Flexirings™ in the stripping sectionand 6' 3" of one inch Flexirings™ in the rectification section. Aredistributor was located midway in the depth of the stripping section.

The tower was modified for process condensate feed at the top of thestripping section (column midsection) and reflux to the top of therectifying section. One inch Flexirings™ were used for tower packing andthe internals of the columns were provided by Koch Engineering. Theseinternals include packing supports, liquid feed distributors, holddowns, redistributors, and demister. Equivalent packing is alsoavailable such as ballast rings by Glitch or High-Pack™ Packing byNorton.

The reboiler and steam control method remained unchanged when revampingfrom stripping only to combination rectification-stripping. Steam rateto the reboiler and the revamped stripper rectifier configurationremains essentially the same as it was in a stripping onlyconfiguration.

From the foregoing it should be understood that this invention is onewell adapted to attain all ends and objects herebefore set forthtogether with the other advantages which are obvious and which areinherent to the structures and apparatus.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other feature andsubcombinations. This is contemplated by and is within the scope of theamended claims.

Since many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth or shown in the accompanying drawing is to beinterpreted as illustrative and no in a limiting sense.

What is claimed:
 1. A method for treating a chemical production plantprocess effluent stream of dilute contaminants, said method comprisingthe following steps:stripping at least a portion of contaminants from aprocess effluent obtained from a chemical production plant, wherein saidstripping is conducted in a stripping section to obtain acontaminant-rich vapor stream and an aqueous bottom stream of reducedcontaminant content; rectifying said contaminant-rich vapor stream in arectification section to obtain a concentrated overhead stream;condensing said concentrated overhead stream to obtain contaminant-richoverhead condensate and non-condensed gases comprising trace amounts ofthe contaminates; scrubbing said non-condensed gases so as to recover aportion of said trace amounts of the contaminates into saidcontaminant-rich overhead condensate; recycling said overhead condensateto said rectifying step to form a closed loop so as to build up theconcentration of contaminants in said overhead condensate; separatingand withdrawing from said closed loop a fraction of said overheadcondensate; and reusing said fraction of the overhead condensate in thechemical production plant.
 2. A method according to claim 1, whereinsaid method additionally comprises collecting said aqueous bottom streamof reduced contaminant content and reusing said bottom stream in saidchemical production plant.
 3. A method according to claim 2, whereinsaid aqueous bottom stream is used as scrubbing water for said scrubbingstep.
 4. A method according to claim 1, wherein said stripping sectionand said rectification section are housed one above another respectivelyin a common upright tower permitting vapor from said stripping sectionto pass upwardly within the common tower to the rectification sectionabove and condensate from each section to pass downwardly within thecommon tower.
 5. A method according to claim 1, wherein said strippingsection and said rectification section are provided in separate columnsin series.
 6. A method according to claim 5, wherein said rectificationstep is conducted in the rectification section to obtain theconcentrated overhead stream and a second aqueous bottom stream ofreduced contaminant content, said second aqueous stream being returnedto said stripping step.
 7. A method according to claim 1, wherein saidreusing step comprises injecting said fraction of overhead condensatewithdrawn from said closed loop back into said chemical productionplant.
 8. A method according to claim 7, wherein said reusing stepcomprises pumping said fraction of overhead condensate under pressurefor entry into said plant.
 9. A method according to claim 1, whereinsaid separating and withdrawing step comprises separating andwithdrawing from said closed loop a fraction of said overhead condensatein an amount ranging from 3 to 50% by volume of the overhead condensatein said loop.
 10. A method according to claim 9, wherein said separatingand withdrawing step comprises separating and withdrawing from saidclosed loop a fraction of said overhead condensate in an amount rangingfrom 5 to 20% by volume of the overhead condensate in said loop.
 11. Amethod according to claim 1 wherein said stripping section is operableat pressures ranging from 0 psia to 300 psig.
 12. A method according toclaim 1, wherein said chemical production plant is selected from thegroup consisting of an ammonia production plant and a methanolproduction plant.
 13. A method according to claim 12, wherein saidfraction of the overhead condensate is delivered to the chemicalproduction plant by delivering said fraction to a mixed feed coil of aprimary reformer of said plant.
 14. A method according to claim 12,wherein said fraction of the overhead condensate is delivered to thechemical production plant by delivering said fraction to an air preheatcoil of a secondary reformer of said ammonia plant.
 15. A process forstripping ammonia plant process effluent comprised of water andcontaminants, said method comprising the steps of:feeding ammonia plantprocess effluent from an ammonia plant to the upper stages of arelatively low pressure steam stripping tower equipped with a bottomcollection basin and operating at pressures ranging from 0 psia to 300psig; allowing said effluent to flow downwardly through a strippingsection of said stripping tower against an upward flow of steam toobtain a contaminant-rich vapor stream, and an aqueous bottom stream ofreduced contaminant content; rectifying said contaminant-rich vaporstream in a rectification zone such that contaminants from the effluentpass upwardly through the rectification zone to form an overhead vaporand condensate formed within said rectification zone is passeddownwardly and returned to said stripping section; condensing saidoverhead vapor to form condensed overhead and non-condensed gasescomprising trace amounts of the contaminates; scrubbing saidnon-condensed gases in a manner to recover a portion of said traceamounts of the contaminates into the condensed overhead; continuouslyrecycling said condensed overhead as reflux to said rectification zoneto form a closed loop which acts to build up a concentration of thecontaminants in said condensed overhead; continuously separating andwithdrawing from said closed loop a fraction of the concentratedcondensed overhead;and reusing said fraction of said condensed overheadin said ammonia plant.